1. Field of the Invention
The present invention relates to a position sensor for sensing a position of, for example, a wheel of a vehicle, such as an automobile, a bus, a truck, a tractor, etc., or a position of a rudder or propeller system of a boat, airplane, etc.
2. Description of the Background Art
A steering system is a major system required for driver operation of a vehicle to control a path of the vehicle. Typically, a driver uses a steering wheel that turns a steering shaft. The steering shaft is usually connected through a rack and pinion gear and additional mechanical linkages, such as a tie rod, to the front wheels. The angle between the front wheel plane and the longitudinal axis of the vehicle is commonly referred to as a wheel angle or a steering angle. This wheel angle is proportional to the rotational angle of the driver's steering wheel.
Some vehicles may be designed with a rear wheel steering system that compliments the front wheel steering system. Because the vehicle operator cannot independently operate two steering systems, obviously, the front and rear wheel steering systems must be coordinated, which is preferably performed via an electronic control systems.
A schematic illustration of a vehicle four-wheel steering system is illustrated in FIG. 1, which is described in U.S. Pat. No. 6,580,988. The vehicle 1 includes front wheels 3 and rear wheels 5. The front wheels 3 are steered together using a known gear and linkage mechanism that can be coupled with a power steering system. The front wheels 3 are connected by a tie rod 7, which is attached to control arms (not shown) of the wheels. The vehicle operator steers the front wheels 3 using a steering wheel 9 attached to a steering shaft 11, which is connected to the tie rod 7 by a linkage system (not further shown), for example, a rack and pinion.
A rear wheel steering mechanism is schematically illustrated at 13, which includes a reversible electrical motor drive that actuates a tie rod (not shown) that is operatively connected to the rear wheels 5. The steering of the rear wheels 5, however, is not under the direct control of the vehicle operator. A rear wheel steering controller 15, which receives data concerning the position of the steering wheel 9 from a position sensor 17 that is provided somewhere on the steering shaft 11, controls the rear wheels 5.
As noted above, conventional steering position sensors are typically provided on the steering shaft of a vehicle and must be able to determine at least a 360 degree rotation of the steering shaft, because, as is commonly known, in order for a vehicle operator to steer the wheels to their maximum steering angle, the steering wheel has to be turned through several 360 degree rotations.
U.S. Publication 2003/0145663, which corresponds to DE 100 41 095 A1, discloses a device for measuring an angle by magnetic sensors or optical sensors. This position sensor, however, must still be fashioned to the steering shaft of a vehicle.
The prior art devices, however, are not able to determine the precise wheel angle of a vehicle wheel because they do not take into account mechanical play of the linkages of a vehicle between the steering shaft and the wheels. Further, this mechanical play increases with wear and tear on a vehicle. Thus, the accuracy of the wheel position continuously decreases as the mechanical play between the linkages increases.
In addition, in particular for the case of vehicles having four wheel steering, as described above, the only provision for determining a wheel position of the rear wheels is typically performed by a position sensor provided directly on the steering shaft. Thus, a single position sensor mounted on the steering shaft provides the position of all four wheels of a vehicle.
Moreover, in particular for the case of an optical position sensor, the conventional position sensor are susceptible to dirt and debris and must therefore be mounted along the steering shaft within the vehicle compartment or require a substantial enclosure to protect the position sensor from the environment, which, may not be feasible because of limited space about the steering shaft or because of economic reasons.